High-frequency furnace



March 11, 1952 K, PQSTHUMUS 2,589,027

HIGH-FREQUENCY FURNACE Filed Oct. 28, 1946 ATTORNEY Patented Mar. l1,195552 UNITED STATES PATENT OFFICE Application October 28, i946, SerialNo. 765,237

En the Netherlands duly 24, 1945 Section 1, Public Law 690, August S,1946 Patent expires July 24, 1955 3 Claims.

This invention relates to a high-frequency furnace which consists of atube oscillator, where the article to be heated is placed within thegenerator coil, more particularly to a high-frequency furnace forhardening purposes.

When, in a high-frequency furnace referred to, the article to be heatedis taken out of the generator coil, the high-frequency voltage increasesvery strongly owing to the fact that the quality of the generatorcircuit has increased.

strong. If the articles to be heated have to stay for a comparativelylong time in the coil this readjustment does not constitute a seriesdrawback in general. Of course, control members must be available. Withfurnaces for hardening purposes, however, the article to be treated iskept in the coil for a very short time, usually for a few seconds only.In this case, it is consequently very objectionable to control thevoltage or, as the case may be, to out it in and out or to controleither any of the circuit elements or the coupling, because this controlthen takes more time than the thermal treatment itself, thus workingvery ineffectively.

By the present invention this drawback is met by interposing a diode inthe grid circuit of an oscillator tube of a high-frequency furnace, thecathode of which is connected to the grid and the saturation current ofwhich is equal to the maximum permissible grid current.

Ii there is an article in the coil, the diode transmits the grid currentwithout undue voltage losses. When the article is withdrawn yfrom thecoil the grid current cannot surpass the maximum value despite anincrease in high-frequency voltage. The grid receives a stronglynegative voltage, so that the anode current and consequently also thehigh-frequency voltage of the circuit do not exceed undue values. Theadvantage over all other controls consists in that this method isentirely devoid of inertia and in that no additional control membersneed be available, it being only necessary that the diode in the gridcircuit should be capable of withstanding a proper voltage.

True, it is alreadyvknown with transmitters to insert a diode in thegrid circuit of an oscillator tube to prevent the grid current fromreversing its sign or from acquiring an unduly high value, but in thesearrangements the dio-cle is only started up by disturbances which wouldcause a reversal or abnormal increase of the grid current and not, aswith the aforesaid high-freuency furnace, by changing the load at anyone une.

In order that the invention may be clearly understood and readilycarried into effect, it will now be described more fully with referenceto the accompanying drawing, given by way of example, which represents ahigh-frequency oscillator for a high-frequency furnace according to theinvention.

In the drawing the oscillator tube of a highfrequency furnace is denotedby I, the cathode by 2, the grid by 3 and the anode by ll.

The generator circuit consists of a condenser and a coil 5. The grid isconnected to the generator circuit through a diode l, which is bridgedby a condenser 3. The grid is connected to the cathode of the diode. Thecathode of the triode f is fed from the low tension mains through theintermediary of a transformer iQ, whereas the anode is supplied withlow-frequency alternating current through the terminals i3. Within thegenerator coil 6 is placed a steel shaft Si which has to be hardened, towhich end it must be kept in the coil for two seconds. When the shaft liis inside the coil (i, the quality of the circuit is poor and thehigh-frequency voltage keeps within the permissible limits. If, however,the shaft is withdrawn, the quality of the circuit increases. Thehigh-frequency voltage increases and consequently also the grid current,but only to the saturation current of the diode l. The negative gridvoltage increases, so that the anode current and consequently thehigh-frequency voltage can no longer increase.

What I claim is:

l. A high-frequency oscillator comprising an electron discharge tubehaving a cathode, a grid i an anode, a resonant circuit coupled betweensaid anode and said grid, and a diode discharge device having a cathodeand an anode, said device being interposed between said grid and circuitwith the cathode of said device con...oted to said grid and the anodethereoic ected to one end of said circuit, said device characterized bya value of saturation current which corresponds to the maximumpermissibl grid current of said tube.

2, .Zi high-frequency oscillator comprising an being connected inparallel with capacitor'with the cathode of said device being connectedto the grid of said tube and the anode of saidA device be'- ingconnected to said other end of saidcircuit.

said device being characterized `by a value of" saturation current whichcorresponds` tot. the maximum permissible grid current of-'saidtube.

3. A high-frequency 'oscillator nforV induction heating comprising anelectron discharge tube having a cathode, a grid and an anode, aresonant circuit constituted by an induction heating coil in parallelwith a condenser, one end of said circuitzbeingiconnectedto the# anodeofzsaidftube, al. diode: discharge device having a/ cathode"A and:

anr anode;.the' cathode: ofVv said,k device being connectedtotheigridrof said'tube, the anode of'said device-being connected. to theotherY end of saidr circuit; al coupling capacitorishunted across saiddevice;` andimeanslto applyfa` low-frequency exi-- 4 ergiaingv potentialbetween the cathode of said tube and a tap yon said coil, said devicebeing characterized by a value of saturation current which correspondsto the maximum permissible 'grid current of said tube.-

KLAAS POSTHUMUS.

REFERENCES CITED The following references are of record in the le ofthis patent.:

UNITEDv STATES PATENTS Number Name Date 1,551,822 Grant Sept. 1, 19251,839,481 Holst et al Jan. 5, 1932 2,113g956. Ebbighausen Apr. 5, 19382;175,694 Jones Oct. 10, 1939 2,304,958u Rfouy Dec. 15, 1942 2,320,876 yMarby June 1, 1943 2,351,4391 Livingston June.13, 1944 2,391,085.vCrandell Dec. 18, 1945. 2,408,229 Roberds. Sept. 24, 19,461. 2,414,141Denneenxet a1. ,Jan.v 14, .1947 2,415,799 Reifeltetfal. Feb.4 .1,1,,194.7 2,420,857 Brown May 201947 2,450,623- Anderson: Oct. 5,1948

